Project Summary/Abstract. Adeno-associated virus (AAV) micro-dystrophin (dys) gene therapy shows great promise to cure Duchenne muscular dystrophy (DMD). Recent studies have shown that DMD patients have a pre-existing immunity or develop immunity against the AAV-dys gene therapy after administration. Although early trial data indicate that systemic gene therapy leads to widespread expression of dystrophin in muscle, the long-term consequence of AAV and dystrophin immunity on the efficacy and stability of gene therapy in not known. Our long-term goal is to determine the role of immunity in AAV-dys gene therapy and define the regulatory mechanisms constraining these immune responses. These discoveries will determine if AAV- and dystrophin-specific immune responses impede on the success of gene therapy and will facilitate the design of potential therapies that mitigate these responses. The objective of the current study is to specifically define whether AAV capsid- and dystrophin- specific T cells limit the efficacy and stability of AAV dys gene therapy, and determine if regulatory T cells (Tregs) mitigate these immune responses in the mdx mouse model of Duchenne muscular dystrophy (DMD). We will test the central hypothesis that Tregs suppress AAV- and dystrophin-specific T cells, which we predict limit the efficiency of myofiber transduction and immunologically ?attack? dystrophin+ myofibers. The premise for the proposed studies is supported by several reported lines of evidence, including that 50% of DMD patients harbor dystrophin-specific T cells, humans develop AAV-specific immunity (humeral and cellular) upon exposure, and Tregs are elevated in dystrophic muscle and suppress antigen-specific T cells. Our hypothesis will be tested by addressing two specific aims: (aim 1) determine the function of AAV- and dystrophin-specific T cells, and (aim 2) determine whether Tregs suppress AAV- and dystrophin-specific T cells. Our proposed study is significant as it will define the functional role of immunity in AAV-dys gene therapy and provide critical insight that will positively impact the design of future gene therapy trials in DMD. The proposed research is also innovative because we will investigate the capacity of Tregs to suppress AAV and dystrophin immunity, a previously- unexamined process. Moreover, we will test the capacity of a humanized IL-2/anti-IL-2 antibody complex (hIL- 2c) to expand Tregs in vivo, and determine its effect on suppressing immunity and the efficacy of AAV dys gene therapy. Insight into Treg-mediated control of AAV- and dystrophin-specific T cells is impactful as it may be leveraged as a unique therapeutic avenue to inhibit AAV- and dystrophin-specific T cells and induce long-term tolerance to AAV or dystrophin.